Generally, while the alloy casing is in the process of solidifying, it possibly gives rise to a defect of shrinkage cavity due to volumetric shrinkage. The casting in the process of solidifying begins cooling from the surface and forms the finally solidified part in the central part of wall thickness. In this central part, the liquid phase that awaits solidification is attracted in the direction of the formerly solidified surface part and, as a result, disposed to induce volumetric shrinkage. This defect of shrinkage cavity varies in form with the composition of the relevant alloy, the condition of cooling, etc. Particularly, in the case of such a copper alloy that tends to induce solute segregation (deviation of concentration) and exhibits a wide range of solidifying temperature, the defect possibly occurs in the form of minute shrinkage holes (shrinkage cavity) called microporosities. The technique which crystallizes a low melting metal or intermetallic compound in an alloy with a view to suppressing the occurrence of this defect and securing the pressure resistance expected in ordinary plumbing materials, such as valves, cocks and joints has been known to the art.
In the bronze casting (CAC406 JIS), for example, lead is added and crystallized as a low melting metal. The CAC406 contains about 5% of lead in weight ratio. Since this lead functions to fill up shrinkage holes occurring in the central part, it permits easy production of such a sound casting that does not have many casting defects like shrinkage cavities. Since this casting excels particularly in machinability, it is copiously utilized for liquid-contacting metal parts in the plumbing materials of the kind under discussion. When this bronze alloy is used as the raw material for liquid-contacting metal parts, such as valves, however, the lead that scarcely forms a solid solution in the bronze casting and manages to crystallize has the possibility of eluting into the ambient water and deteriorating the quality of the water. This phenomenon becomes particularly conspicuous when water stagnates in the liquid-contacting metal part.
Thus, the development of the so-called leadless copper alloy is being promoted and has succeeded in proposing several new alloys (refer, for example, to Patent Documents 1 to 4).
JP-B HEI 5-63536 (Patent Document 1), for example, discloses a leadless copper alloy that is enabled by adding Bi in the place of lead in the copper alloy to enhance machinability and prevents dezincification.
Japanese Patent No. 2889829 (Patent Document 2) discloses a leadless bronze which adds Bi for the sake of enhancing machinability and adds Sb to suppress the occurrence of porosities during the course of casting and enhance mechanical strength.
JP-A 2000-336442 (Patent Document 3) discloses a leadless free-cutting bronze alloy that acquires machinability and enhances anti-seizing property by adding Bi and secures resistance to dezincification and mechanical properties by addition of Sn, Ni and P.
JP-A 2002-60868 (Patent Document 4) discloses a leadless bronze alloy which is enabled by adding not more than 1 weight % of Bi and Sb as impurities and taking into consideration the recycling property to secure castability, workability and mechanical properties.
Patent Document 1: JP-B HEI 5-63536
Patent Document 2: Japanese Patent No. 2889829
Patent Document 3: JP-A 2000-336442
Patent Document 4: JP-A 2002-60868